Method and apparatus for improving the frequency resolution of a direct digital synthesizer

A method for increasing the frequency resolution of a direct digital synthesizer (DDS) to produce a high purity reference signal, comprising:

• selecting N frequencies F1 through FN to be generated in a pattern of frequencies that repeats with a period of duration T, and selecting N time durations T1 through TN as the durations of F1 though FN respectively during T such that T1+T2+ . . . +TN=T and an absolute value of the largest frequency difference between any two of the frequencies F1 though FN is at least two orders of magnitude less than π/T;
• selecting N frequency tuning words FTW1 through FTWN representing a phase increment required at each clock period to generate F1 through FN, respectively;
• applying FTW1 through FTWN to the DDS for the durations T1 through TN in the repeating pattern of frequencies;
• and
• generating the high purity reference signal based on the repeating pattern of frequencies and FTW1 through FTWN.

An apparatus for generating a high purity reference signal with high frequency resolution, comprising:

• a first storage device for storing N frequency tuning words FTW1 through FTWN representing a phase increment required at each clock period to generate N frequencies F1 through FN, respectively;
• a first storage device controller coupled to the first storage device for controlling the first storage device to generate FTW1 through FTWN in a pattern of frequencies that repeats with a period of duration T, where T1 through TN represent the durations of FTW1 through FTWN respectively during T such that T1+T2+ . . . +TN=T and an absolute value of the largest frequency difference between any two of the frequencies F1 though FN is at least two orders of magnitude less than π/T;
• a phase accumulator coupled to the first storage device for receiving the generated FTW1 through FTWN and accumulating a total accumulated degree of phase at each clock period;
• and
• a second storage device coupled to the phase accumulator for converting the total accumulated degree of phase to generate a representation of the high purity reference signal at each clock period.